Visual display manufacturers must simultaneously satisfy several, often competing, requirements. These include the need to minimize the number of individual light sources used, constraints on overall system physical dimensions, and the attainment of high degrees of both color and brightness uniformity. Achieving color and brightness uniformity requirements are particularly complicated by the fact that a single visual display (e.g., automobile instrument panel, cockpit display, signage display) may utilize a mixture of different source types (e.g., incandescent bulbs, light-emitting diodes (LEDs), and/or fluorescent lights) having significantly different spectral characteristics. Accurate calorimeter measurement (i.e., determining luminance and chromaticity coordinates) of such a display can provide several significant challenges.
One conventional process for measuring such displays includes the use of a single luminance meter and/or colorimeter (i.e., a “spot photometer” or “spot colorimeter,” respectively) to measure the different portions of the display having different spectral characteristics. This conventional approach can generally provide the necessary measurement with a sufficient degree of accuracy, but this process is far too slow for production applications and, in many cases, most research uses.
Another approach for measuring such displays includes the use of an imaging photometer (e.g., a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) imaging system). A CCD system, for example, can substantially improve the speed and efficiency of testing displays because a CCD array can include a large number of detectors and, accordingly, a single measurement with the CCD system can include a very large number of test points (e.g., in excess of 1×106). Rather than taking many separate spot measurements of a visual display (as necessary with the above-described spot detectors), the CCD system can image the entire display simultaneously and capture many thousands or millions of points in just seconds.
Measuring displays with CCD systems, however, also includes several drawbacks. For example, the transmission of the filters in combination with the camera response generally does not exactly match the tristimulus curves. Accordingly, an accurate color calibration can generally be established for a small color range, but would not be accurate for the entire visible spectrum. This is particularly true of a display that has several different colors using narrow band light sources, such as LEDs in an instrument cluster. One conventional method to overcome this problem is to make several color calibrations and take several measurements to determine the correct chromaticity and luminance values. This method, however, is extremely time consuming and requires significant computing resources (e.g., for post-processing of the data and storage of the large amounts of data). Accordingly, there is a need to improve the systems and methods for measuring visual displays.